Baseband Tx Secondary Filter; Rx Tia Calibration Equations; Rx Adc Setup; Baseband Dc Offset Calibration - Analog Devices AD9361 Reference Manual

AD9361 Reference Manual

BASEBAND Tx SECONDARY FILTER

The baseband Tx secondary filter is a tunable single pole filter
after the baseband Tx analog filter. The Tx secondary filter
corner is ideally set to 5 times the baseband bandwidth to help
filter out of band Tx noise emissions. This filter is programmed
when the ad9361_set_tx_rf_bandwidth function is called.

Rx TIA CALIBRATION EQUATIONS

The Rx transimpedance amplifier (TIA) is located between the
mixer and Rx baseband analog filter. The TIA has two gain
settings (0 dB gain and −6 dB gain), and applies a single pole
filter with a corner at 2.5× the baseband bandwidth. The
corner frequency of the Rx TIA is programmed when the
ad9361_set_rx_rf_bandwidth function is called. When the gain
index in the AD9361 Rx gain table changes the Rx TIA gain
setting, the AD9361 device will automatically scale the amplifier
to maintain the same corner frequency.

Rx ADC SETUP

The receive ADC is a third-order continuous time delta-sigma
modulator and is highly programmable. The values in many of
the ADC registers change with sampling clock frequency while
others do not change but the correct initial settings are critical
for proper operation. The ADC is programmed with the
ad9361_set_rx_rf_bandwidth function.

BASEBAND DC OFFSET CALIBRATION

It is recommended to run the baseband DC offset calibration
once during device initialization in the ALERT state. Since the
baseband signal path does not change with different wireless
standards or clock frequencies, it should not need to be run
again. The baseband DC offset correction values are stored for
all of the Rx analog baseband filter gain steps. The correction
words are applied as the Rx gain changes based on the current
Rx gain table index. This calibration is run during the
ad9361_bb_dc_offset_calib function call.
The one-shot baseband DC offset calibration will complete in a
finite time depending on the device settings. The calibration
time is 404,000 ClkRF cycles.

BASEBAND DC OFFSET TRACKING

Baseband DC offset tracking should be used in conjunction
with the RF DC offset tracking option. A high-pass filter loop is
utilized to track DC changes caused by the RF DC offset and Rx
quadrature correction block. The speed and accuracy of the BB
DC tracking loop are configurable. Also, an option exists for the
BB DC offset tracking to attack/settle quickly after a gain
change and then after a certain time switch to slower DC offset
tracking updates. When using the tracking mode, first run an
initial one-shot baseband DC offset calibration to minimize any
static DC offsets in the signal chain. Then enable tracking along
with the desired tracking loop settings. Baseband DC offset
tracking is enabled in the ad9361_tracking_control function
call.
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